This invention relates to an image forming method and apparatus for forming an image by ejecting a printing fluid such as ink onto a printing medium while causing printing means to scan a printing medium.
The inkjet printing method, which allows printing to be performed on a variety of printing media, makes possible high-density, high-speed printing. For this reason, the method has been applied to full-size printers and portable printers used to produce the output of various devices, and such printers have appeared as commercial products. Each individual printer is adapted to support the functions and mode of use specific to the particular device.
In general, an inkjet printer is equipped with a carriage on which printing means (a printhead) and an ink tank are mounted, transport means for transporting the printing medium, and control means for controlling these components. The printhead, which ejects ink droplets from a plurality of orifices, is caused to perform serially scanning in a direction (referred to as the xe2x80x9cmain-scan directionxe2x80x9d) at right angles to the direction (referred to as the xe2x80x9csub-scan directionxe2x80x9d) in which the printing paper is transported. When printing is not being performed, the printing medium is transported intermittently in increments equivalent to printing width. This printing method is such that printing is carried out by ejecting ink onto the printing paper in accordance with a print signal. The method is in widespread use owing to its low running cost and quietness. In recent years a large number of products in which this method is applied to color printers using multiple color inks have come into practical use.
Color printers to which the inkjet printing method is applied have printheads whose structures can be broadly classified into two different types.
The first type is a printhead in which a large number of nozzles for ejecting ink are placed on a straight line extending in the sub-scan direction, as shown in FIG. 15. Here a printhead 100 has nozzles 100y, 100m, 100c, 100k for ejecting yellow, magenta, cyan and black ink, respectively, arranged in a single row in the sub-scan direction in such a manner that colors will not overlap. FIG. 16 illustrates an arrangement if a printhead 101 in which the nozzle 101k for ejecting black ink is disposed separate from the nozzles 101y, 101m, 101c that eject color ink. It should be obvious from FIGS. 15 and 16 that images in the colors yellow, magenta and cyan are formed at different positions on the printing paper in a single main scan of the printhead. If so-called secondary colors of blue, red and green are formed, therefore, the sequence in which colors are superimposed is fixed irrespective of the direction in which the printhead is scanned. For example, if a blue image is formed, first cyan is printed and then magenta is printed so as to overlap cyan. Accordingly, if the printhead 100 or 101 is used, unevenness in color will not occur even when printing by back-and-forth scanning of printhead is performed.
However, if the number of nozzles of each color is increased in order to raise printing speed, the length of the printhead is enlarged, resulting in an apparatus of greater size. There is also a tendency toward greater complexity in terms of retaining the printing medium in the printing area of the printhead, thereby inviting an increase in the cost of the printhead and apparatus. Accordingly, a printhead of a second type, described below, has come into use.
As shown in FIG. 17 by way of example, a printhead 102 of the second type has printheads 102k, 102c, 102m, 102y, which eject black, cyan, magenta and yellow ink, respectively, arrayed in the main-scan direction. If the printhead 102 is used, the inks of all colors are ejected by a single scan in accordance with image data.
When an image is formed by alternately repeating a scan in the left-to-right direction (the direction indicated by arrow A in the drawings) and in the right-to-left direction (the direction indicated by arrow B) in order to raise printing speed, the sequence in which the colors are superimposed in the scanning of printhead 102 in the A direction differs from that of the B direction if a secondary color of blue, red or green, for example, is formed. As a consequence, the hue differs in each scan and color unevenness results, causing a major decline in image quality.
A multiple-pass printing method the purpose of which is to improve image quality is available. This method causes printing to be performed at the same location by two or more scans, thereby mitigating nozzle-to-nozzle variance peculiar to a printhead. In this case, the distance traveled by the printing medium each time it is transported is half the length of the head. Printing time can be reduced by using back-and-forth printing also when forming an image by this method. However, when a secondary color is formed, color unevenness caused by a difference in the sequence of color overlap occurs even with this printing method.
As mentioned above, there are two types of structures for inkjet printheads applied to a color printing apparatus. However, the type that can be said to be suited to raising the speed of printing of the apparatus is the second type, namely the type in which the nozzles of each color are arranged in the scanning direction of the printhead. In-the case of the second type, however, a problem which arises is the scan-to-scan color unevenness caused by a difference in which colors overlap in the scanning of the printhead in the back and forth directions, as mentioned above.
In order to solve this problem, a method of andU changing the method of color processing so that one method is used when the printhead is scanned in the left-to-right direction and another method when the printhead is scanned in the right-to-left direction has been disclosed in the specification of Japanese Patent Publication No. 3-54508. However, this method requires two types of color processing tables for the respective scanning directions.
In a case where printing is performed by an ordinary printer, color processing is executed by a printer driver operating in a host computer and image data that has undergone color processing is sent to the printer proper. The printer performs printing by manipulating and processing the received image data in conformity with the printhead. However, when color processing is executed in the arrangement described in the above-mentioned specification, it is necessary to ascertain beforehand how the image data being processed will be printing on the printer side. For example, if color processing is changed between left-to-right and right-to-left directions when printing is carried out by back-and-forth scanning, the host computer side must recognize whether the printhead will print by a scan in the left-to-right direction or by a scan in the right-to-left direction. However, this results in a very highly complicated system, inclusive of the printer driver and printer proper.
Furthermore, the color reproduction ranges (hues) of the scans in the left-to-right and right-to-left directions usually deviate from each other. This means that if the color processing is changed so as to achieve a common color reproduction range for both of the scanning directions, the color reproduction range will become narrower and may bring about a decline in image quality.
Accordingly, an object of the present invention is to prevent a decline in image quality caused by a difference in coloring sequence between back and forth scans, thereby making it possible to achieve high-quality printing of an image.
Another object of the present invention is to address the problem of color unevenness due to a difference in color ejection sequence caused by performing back-and-forth printing using a printhead in which the nozzles of each color are arranged in the direction of scanning, and the problem of a decline in printing speed caused by performing printing in one direction only, thereby to make possible an improvement in printing speed while reducing color unevenness even in a color printing apparatus in which the sequence of ink ejection differs between back-and-forth scans.
According to the present invention, the foregoing objects are obtained by providing an image by repeatedly causing a printhead, which prints an image, to print on and scan a scan area using printing materials of a plurality of types, comprising: acquisition means for quantifying the state of a mixture of the printing materials in printing of an image in the next scan area, thereby acquiring index information representing the state of the mixture; decision means for deciding direction of a printing scan in to the next scan area based upon the index information acquired by the acquisition means; and print execution means for executing a printing scan in the next scan area by performing a printing scan in the direction decided by the decision means.
Further, according to the present invention, the foregoing objects are attained by providing an image forming method of forming an image by repeatedly causing a printhead, which prints an image, to print on and scan a scan area using printing materials of a plurality of types, comprising: an acquisition step of quantifying the state of a mixture of the printing materials in printing of an image in the next scan area, thereby acquiring index information representing the state of the mixture; a decision step of deciding direction of a printing scan in to the next scan area based upon the index information acquired at the acquisition step; and a print execution step of executing a printing scan in the next scan area by performing a printing scan in the direction decided at the decision step.
Other features and advantages of the present invention will be apparent from the following description taken in conjunction with the accompanying drawings, in which like reference characters designate the same or similar parts throughout the figures thereof.